Determining Energy Demand: Wh from Load and Runtime
The first step is to determine the energy demand in watt-hours (Wh). The basic formula is: Wh = V × Ah — equivalently: Wh = W × h. For an application with an average consumption of 500 W running for 4 hours per cycle, the energy demand is 2,000 Wh.
In practice the load is rarely constant. An automated guided vehicle (AGV) draws several times the running current briefly when accelerating, then operates at partial load, and stands still between jobs. For sizing purposes, the relevant figure is therefore not the peak current but the average energy consumption per cycle — derived from a load profile or from measurements on a reference system. If no load profile is available, adding 20–30 % on top of nominal consumption is a reasonable starting point; this margin is later replaced or supplemented by the DoD reserve.
For applications with strongly varying load — hoists, welding machines, presses — average power alone is insufficient. The thermal energy (I²t) must also be considered, because high pulse currents thermally stress the pack even when the energy content per cycle is low.
Voltage Level and Cell Configuration: Series (S) and Parallel (P)
The pack's nominal voltage is the number of series-connected cells (S) multiplied by the nominal cell voltage. LiFePO4 cells have a nominal voltage of approximately 3.2 V; Li-NMC cells approximately 3.6–3.7 V. A 48 V pack using LiFePO4 typically requires 15 cells in series (15S × 3.2 V = 48 V nominal). The actual operating voltage swings between fully charged (15 × 3.65 V = 54.75 V) and the discharge cut-off (15 × 2.5 V = 37.5 V). The downstream DC/DC converter or drive inverter must tolerate this voltage swing.
The capacity in Ah is the number of parallel cell strings (P) multiplied by the capacity of a single cell. Two parallel strings of 100 Ah each give 200 Ah. Total pack energy is then: Wh = V_nominal × Ah_total. A 48 V / 200 Ah pack delivers 9,600 Wh.
The number of series cells is determined by the system voltage — motor controller, BMS input voltage and charger define the voltage window. The number of parallel strings follows from the energy demand and the C-rate requirement: more parallel strings not only increase capacity but also distribute the current across more cells — reducing the per-cell C-rate and extending service life.
Specifying C-Rate, Continuous Current and Peak Current
The C-rate describes the charge or discharge current relative to the rated capacity: C-rate = current [A] / capacity [Ah]. For a 100 Ah pack drawing 100 A, the C-rate is 1C — the pack would theoretically be fully discharged in one hour. At 200 A it is 2C and 30 minutes theoretical runtime.
Two current values are relevant for sizing: the continuous discharge current the pack can sustain indefinitely without thermal overload, and the peak current permitted briefly (typically seconds to minutes). Both are chemistry-dependent and specified by the cell manufacturer in the datasheet. LiFePO4 cells generally tolerate higher continuous currents than Li-NMC cells of equal capacity because their thermal stability is greater. Li-NMC offers higher energy density in return.
A common sizing error: the application's peak current is used as the continuous current. Correct practice: determine the continuous current at the typical operating point; verify the peak current (e.g. start-up inrush) separately against the cell's permissible pulse current. The BMS limits current in both cases and protects the cells — but it does not replace correct sizing that leaves adequate thermal headroom.
Planning Reserves: DoD, Ageing and Temperature
A cell's rated capacity is a laboratory figure at defined conditions (typically 25 °C, 0.2C discharge). In real operation usable capacity is lower — for three reasons:
First, depth of discharge (DoD): a pack that is always discharged to 0 % ages significantly faster than one that stops at 20 % remaining charge. Best practice is to limit the usable range to 80–90 % DoD, planning a 10–20 % capacity reserve at the bottom. The BMS enforces the corresponding discharge cut-off at cell voltage level.
Second, ageing: lithium cells lose capacity with every cycle and over time (calendar ageing). A pack that must still deliver 80 % of its initial capacity after 1,000 cycles must be sized larger from the start — or the application must accept decreasing runtime. As a rule of thumb: LiFePO4 with moderate C-rate and 80 % DoD can reach 2,000–3,000 cycles to the 80 % threshold; Li-NMC is typically lower depending on the design.
Third, temperature: at low temperatures usable capacity drops noticeably. At 0 °C a LiFePO4 pack's available capacity can be 10–20 % below the 25 °C value; at −20 °C the drop is considerably larger. Applications outdoors or in cold stores must account for this in the sizing and may need to include active heating.
Typical Sizing Mistakes
Mistake 1 — Rated capacity equated with usable capacity: An application needing 10 kWh and ordering a 10 kWh pack will get less under real conditions. DoD reserve, ageing margin and temperature derating add up easily to 30–40 %. A 10 kWh demand typically requires 13–15 kWh rated capacity.
Mistake 2 — Voltage swing not accounted for: A 48 V pack (LiFePO4, 15S) delivers 54.75 V when fully charged and 37.5 V when discharged. Motor controllers and converters rated only to 48 V will be overvoltaged when the pack is full. The complete voltage range — from fully charged to cut-off — must feed into the system design.
Mistake 3 — Cell C-rate ignored: Standard 18650 cells for consumer electronics are designed for 0.5C. Running them at 2C continuous in an AGV causes overheating, rapid ageing and safety risk. Only cells rated for the required C-rate should be used — check the datasheet under "continuous discharge current".
Mistake 4 — BMS treated as a sizing substitute: The BMS protects the pack from limit violations but cannot compensate for a structurally undersized pack. A pack operated continuously at its thermal or electrical limits ages prematurely — even if the BMS never trips.
Battery Pack Sizing Checklist
- ✓Energy demand in Wh derived from load profile or measurements — not estimated?
- ✓Voltage level defined — fully charged voltage, nominal voltage and cut-off voltage aligned with the system electronics?
- ✓Continuous and peak discharge current determined separately and cross-checked against the cell datasheet?
- ✓DoD reserve included (typically 10–20 % remaining capacity as the lower limit)?
- ✓Ageing margin considered — sizing based on end-of-life capacity after target runtime, not initial capacity?
- ✓Operating temperature range accounted for — capacity derating and charge current limits at low temperatures factored in?
- ✓BMS limits matched to the chosen cell chemistry and configuration (voltage window, current limits, thermal thresholds)?
The first three points establish the basic pack size. Points four to six determine whether the pack is sufficient throughout its entire service life and under all operating conditions. If you are uncertain what capacity margin is appropriate for your application, we are glad to work through it based on your specification.